Because acrylic adhesive compositions conventionally used for polarizing plates have weight-average molecular weights (typically 1,000,000 to 1,500,000) higher than the weight-average molecular weights of acrylic adhesive compositions for labels and building materials, coating of these acrylic adhesive compositions for polarizing plates requires large amounts of diluting solvents (such as toluene and ethyl acetate) and accordingly requires a long drying time to remove the diluting solvents, which results in low production efficiency. An amendment of the Air Pollution Control Act has tightened the control of emissions of volatile organic compounds (VOC) such as toluene and ethyl acetate used as the diluting solvents. In view of the situations described above, there is an increasing demand for an adhesive composition for polarizing plates that can be applied at a high concentration of polymer or a small amount of diluting solvent.
For example, Patent Document 1 (Japanese Patent Laid-Open No. H09-145925) discloses a particular acrylic adhesive composition for polarizing plates that has improved adhesive and cohesive durability. The acrylic adhesive composition contains a thiol compound having an alkoxysilyl or hydroxyl group as a chain transfer agent in a particular acrylic monomer mixture and has a weight-average molecular weight in the range of 800,000 to 1,500,000.
However, a coating solution containing such a high-molecular-weight adhesive composition in an organic solvent has a very high viscosity. When the coating solution is practically applied to a substrate, therefore, the coating solution must be diluted with solvent to a concentration in the range of approximately 15 to 20% by weight. This results in low production efficiency.
Patent Document 2 (Japanese Patent Laid-Open No. 2007-9006) discloses an acrylic adhesive for optical displays that has a viscosity in the range of 1000 mPa·s to 4000 mPa·s. Although such a viscosity allows the amount of solvent used in coating to be decreased, characteristics such as durability and adhesion required for an adhesive composition for polarizing plates were not sufficiently achieved.
Patent Document 3 (Japanese Patent Laid-Open No. 2001-40037) discloses an acrylic polymer produced using a thiol compound in the presence of a particular catalyst, and a curable composition. This acrylic polymer is a polymer which has a controlled polymer end group and is produced using a metallocene catalyst and a thiol compound having a reactive silyl group in the molecule. A high-reactive monomer, the reaction of which could not be controlled previously because of a runaway reaction, can be obtained without using a reaction solvent. The acrylic polymer has further improved reaction curability.
However, the percentage of polymerization ranges from approximately 40 to 60% by weight, and the resulting polymer has a weight-average molecular weight of as low as 500 to 100,000. Characteristics such as adhesion and durability of the acrylic polymer must therefore be improved to use the acrylic polymer as an adhesive for polarizing plates.
As adhesives for polarizing plates, adhesive compositions where an acrylic polymer having a functional group, a cross-linker that can react with the functional group to form a three-dimensional cross-linked structure, and a silane coupling agent are dissolved in an organic solvent are used.
In such adhesives for polarizing plates, a functional group, such as a hydroxyl group or a carboxyl group of an acrylic polymer can be bonded to an isocyanate group of a polyisocyanate compound or an epoxy group of a glycidyl compound which are curing agents, to form a three-dimensional structure, thereby adhering a glass substrate to a polarizing plate.
However, since an adhesive composition for polarizing plates is used for expensive liquid crystal apparatuses, a work-in-process roll coated with an adhesive (a three-layer structure of a polarizing plate/an adhesive layer/a release film, or a release film/an adhesive layer/a release film) is stored in an aging room set at a temperature in the range of 20 to 50° C. (in general, 40° C.) for approximately one week to crosslink the adhesive so as to meet predetermined performance requirements. The work-in-process roll is then cut into a desired size. After a release film is removed, the adhesive layer is attached to a liquid crystal cell. Thus, a large number of work-in-process rolls corresponding to the storage period in the aging room must be stored. In addition, costs associated with the aging room are high. Thus, there is a growing demand for an adhesive composition for polarizing plates that can decrease the curing time and obviate the need for an aging room.
As a method for decreasing the curing time, a curing accelerator is sometimes added to an acrylic monomer. The addition of a curing accelerator, however, often considerably increases the viscosity of an adhesive at an early stage, resulting in an increase in the viscosity of a coating solution. This makes the application of the adhesive difficult.
If such gelation at an early stage has increased the viscosity of an adhesive coating solution, the adhesive coating solution cannot be smoothly applied without using a large amount of organic solvent. As in the high-molecular-weight adhesive coating solution described above, therefore, there is the problem that a large amount of organic solvent must be removed to form an adhesive layer having favorable characteristics.
Curing accelerators to be added to such adhesives described above include various compounds such as amine compounds and organic metal compounds.
For example, Patent Document 4 (Japanese Patent Laid-Open No. 2003-329832) discloses an adhesive optical film having an adhesive layer on one side of the optical film, wherein the adhesive layer is formed of a bridged compound of a composition containing an acrylic polymer having a weight-average molecular weight of 700,000 or more and a functional group that can react with an isocyanate, a polyhydroxyalkylamine compound and a polyisocyanate compound. Although the polyhydroxyalkylamine compound is used as a curing accelerator, such an amine compound turns yellow over time and is therefore unsuited as a curing accelerator for an adhesive for optical films.
Furthermore, for example, Patent Document 5 (WO 2007/058277 pamphlet) paragraphs [0036] to [0038] describe the availability of a cross-linking accelerator to rapidly form a crosslinked structure with an isocyanate compound and provide a listing of specific curing accelerators, including amino compounds such as N,N,N″,N″-tetramethylhexanediamine, triethylamine and imidazole; and organometallic compounds such as cobalt naphthenate, dibutyltin diacetate, dibutyltin diacetylacetonate, tetra-n-butyltin, trimethyltin hydroxide and dibutyltin laurate.
It is an object of the present invention to provide an adhesive composition for polarizing plates that ensures characteristics such as adhesion and durability required for adhesives for polarizing plates and that can be applied as an adhesive with a decreased amount of diluent, and a polarizing plate having an adhesive layer formed using the adhesive composition.
It is another object of the present invention to provide an adhesive composition for polarizing plates that has a pot life suitable for adhesive coating and that can greatly reduce the curing time, and a polarizing plate formed using such adhesive composition.